The present invention relates to a method for deriving the word timing of a pulse position modulated signal in which each word is subdivided into a plurality of time slots and a pulse occurs in any one of the time slots of every word. The phrase "deriving the word timing" as used herein means determining the boundaries of the words in the PPM signal, or where one word stops and the next word begins. The invention also relates to circuitry for implementing the method.
To be able to derive the word timing from a received pulse position modulated signal and to be able to synchronize the demodulator with the word timing, in accordance with a customary method at least one synchronizing word from which the word timing can be derived is inserted into the pulse position modulated (PPM) signal at the transmitting end. At the receiving end, this synchronizing word may then be detected by employing the maximum likelihood method of synchronization as disclosed, for example, by Costas N. Georghiades, "Joint Baud and Frame Synchronization in Direct Detection Optical Communications," IEEE Transactions on Communications, Vol. COM-33, No. 4 (April, 1985) pages 357-360 or by Costas N. Georghiades, "Optimum Joint Slot and Symbol Synchronization for the Optical PPM Channel," IEEE Transactions on Communications, Vol. COM-35, No. 6 (June, 1987) pages 632-636.